1. Field of the Invention
The present invention relates to a hydraulic brake system for automotive vehicles with a hydraulic power booster and a master cylinder connected downstream of the power booster. The pressure chamber of the power booster is pressurized by auxiliary pressure which is dependent on the position of a pedal actuatable brake valve. An auxiliary cylinder comprising a fast-fill cylinder is provided The fast-fill cylinder is pressurized by the dynamic pressure and with the aid of which the working chamber of the master cylinder can be supplied with additional pressure fluid in the initial phase of braking.
2. Description of the Prior Art
In German application P No. 30 16 683.7, for example, a hydraulic brake system is proposed which comprises a tandem master cylinder with a hydraulic power booster connected upstream thereof. One chamber each of the tandem master cylinder communicates with wheel brake cylinders by way of a brake conduit into which a so-called two-way valve is inserted.
A characteristic of the wheel brake cylinders of recent types of vehicle brakes generally is that they consume a great volume of brake fluid before the pad is applied to the brake disc. Such conventional brake systems, wherein an increase of the braking pressure takes place by means of a hydraulic power booster connected upstream of the master cylinder, therefore have the disadvantage due to the great volume of fluid taken in that the master cylinder pistons have to travel a certain distance before any braking effect is achieved at all. Furthermore, the break-off or non-braking effect in this connection results because the booster piston only starts moving when a relatively high pressure has been reached in the pressure chamber of the hydraulic power booster. The frictional forces of the packing material in the transition from static friction to sliding friction decrease abruptly so that the braking pressure in the working chambers of the master cylinder starts to increase more or less abruptly.
In order to avoid this functional disadvantage the noted German patent application provides for the hydraulically actuatable two-way valves which shut off the brake conduits when the brake is in an inactive position. That is, when the working chambers of the master cylinder are in unpressurized condition, while the two-way valves are additionally connected to the pressure of the hydraulic power booster.
It is a known fact that when a pedal-actuated brake valve of the hydraulic power booster is actuated, first a connection between the pressure chamber and an unpressurized supply reservoir is closed, whereas after a control path beyond that point the pressure supply towards the pressure chamber is at first partially opened. Thus, a relatively low pressure builds up at first in the pressure chamber of the hydraulic power booster, which pressure, though not yet sufficient to set the booster piston in motion, reaches the wheel brake cylinders by way of the two-way valves connected to the pressure chamber. In this system, pressure medium under not more than low pressure is led to the wheel brake cylinders, with the consequence that the wheel brake cylinders overcome the idle movements inherent in the system while taking in a certain fluid volume. By adequate dimensioning of the brake system the idle movements are overcome when the pressure in the pressure chamber is just high enough to set the booster piston in motion. Thus, there is a pressure build-up in the working chambers of the tandem master cylinder
Furthermore, the hydraulically actuatable two-way valves according to the known system are designed such that they change over when the pressure in the working chambers of the master cylinder exceeds the pressure in the dynamic brake circuit. At this changeover pressure a free connection between the wheel brake cylinders and the working chambers of the tandem master cylinder is established, whereas the connection between the pressure chamber of the hydraulic power booster and the wheel brake cylinders is interrupted.
In certain applications it is desirable to have a low braking effect achieved at the wheel brakes when the working chambers of the master brake cylinder are still unpressurized. This is advantageous in particular for automotive vehicles with automatic transmission, since such a vehicle can be kept in idle position, even in the drive gear, by applying a relatively low actuating force to the brake pedal.
In the German patent application P No. 32 15 954.4, a hydraulic brake system is known wherein a braking pressure generator is provided for actuating wheel cylinders. The braking pressure generator comprises a hydraulic power booster and a master cylinder connected downstream of the hydraulic power booster. The master cylinder comprises a master cylinder piston with a relatively low diameter, whereas the booster piston has a larger diameter. Inside the booster piston, a brake valve is arranged by means of which a hydraulic pressure is adjustable in the pressure chamber of the hydraulic power booster in dependence of the force applied from time to time to the brake pedal.
Arranged beside the braking pressure generator is an auxiliary cylinder in which a piston is slidably sealingly guided against the force of a pressure spring. The piston guided in the auxiliary cylinder is pressurized at one front surface by the pressure prevailing from time to time in the pressure chamber of the hydraulic power booster, while the chamber accommodating the pressure spring is connected with the working chamber of the master cylinder. Since the piston inside the auxiliary cylinder has a considerably smaller diameter than the diameter of the booster piston, the piston is displace at very low pressures in the pressure chamber of the hydraulic power booster so that a corresponding pre-fill volume is introduced into the brake actuating elements before the booster piston with the master cylinder piston coupled to it is set in motion. Between the pistons of the braking pressure generator an annular chamber is formed in the housing which chamber communicates by way of a corresponding pressure conduit with an unpressurized supply reservoir.
In the described brake system, if certain secondary conditions are met, the piston of the auxiliary cylinder starts moving sooner or later than the booster piston so that the desired pre-fill effect may not be achieved under certain circumstances.